An inkjet printing system is a type of fluid ejection device, and includes a printhead, an ink supply, and an electronic controller that controls the printhead. The printhead ejects liquid ink drops through an array of orifices or nozzles disposed in a die by rapidly heating small volumes of ink located in vaporization chambers. The ink is heated with small electric heaters, such as thin film resistors or firing resistors. Heating the ink causes a portion of the liquid ink to vaporize and thereby eject a single drop through the nozzle toward a sheet of print medium, such as a sheet of paper, to print an image. The ink nozzles are typically arranged in one or more arrays in the printhead die, such that properly sequenced ejection of ink from the nozzles causes characters or other images to be printed as the printhead scans across the print medium.
To eject each drop of ink, the electronic controller that controls the printhead activates an electrical current from a power supply external to the printhead. The electrical current passes through a selected firing resistor to heat the ink in a corresponding selected vaporization chamber and eject the ink through a corresponding nozzle. Known drop generators include a firing resistor, a corresponding vaporization chamber, and a corresponding nozzle.
In inkjet printing systems it is desirable to have several characteristics of each print cartridge easily identifiable by a controller, and it is desirable to have such identification information supplied directly by the print cartridge. This “identification information” can provide information to the controller to adjust the operation of the printer and ensure correct operation. Additionally, as the different types of fluid ejection devices and their operating parameters increase, there is a need to provide a greater amount of identification information without adding further interconnections to the flex tab circuit or increasing the size of the die to provide such identification information.
For these and other reasons, pen identification cells have been developed and integrated with the circuitry of inkjet printhead dies. In one configuration, the printhead circuitry is a negative-channel metal-oxide semiconductor (NMOS) circuit, and the identification cells are configured to be addressed individually. Each identification cell includes an identification bit that stores one bit of information.
The identification bits of the identification cells typically employ fuses and, though they are different from standard programmable read-only memory (PROM) chips, these bits are programmed and used in basically the same way. To program the chip, a relatively high current is selectively routed to certain fuses to cause them to burn out. Bits where fuses remain have a value of 1, while those where the fuses have been burned out provide a value of 0 in the binary logic of the circuit.
Programming and using ROM chips in this way has some drawbacks. If a chip is improperly programmed initially, there is no way to fix it, and the chip must be discarded. Additionally, fuses are relatively large, and can be unreliable. In inkjet printhead circuits, for example, fuses can damage the inkjet orifice layer during programming, and after a fuse burns out, metal debris from the fuse can be drawn into the ink and cause blockage in a pen, or result in poor quality printing.
In recent years, electronically programmable read-only memory (EPROM) devices have also been developed. Unlike PROM chips, EPROM chips do not include fuses. Like typical ROM chips, EPROMs include a conductive grid of columns and rows. The cell at each intersection has two gates that are separated from each other by a thin oxide layer that acts as a dielectric. One of the gates is called a floating gate and the other is called a control gate or input gate. The floating gate's only link to the row is through the control gate. A blank EPROM has all of the gates fully open, giving each cell a value of 1. That is, the floating gate initially has no charge, which causes the threshold voltage to be low.
To change the value of the bit to 0, a programming voltage (e.g. 10 to 16 volts) is applied to the control gate and drain. This programming voltage draws excited electrons to the floating gate, thereby increasing the threshold voltage. The excited electrons are pushed through and trapped on the other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. During use of the EPROM cell, a cell sensor monitors the threshold voltage of the cell. If the threshold voltage is low (below the threshold level), the cell has a value of 1. If the threshold voltage is high (above the threshold level), the cell has a value of zero.
Because EPROM cells have two gates at each intersection, an EPROM chip requires additional layers compared to a standard NMOS or PROM chip, including many such chips that are frequently used in inkjet printhead circuits. Consequently, while some of the drawbacks of fuses in NMOS circuits could be eliminated by the application of EPROM circuitry, the use of a typical EPROM cell either requires that the chip be provided with additional layers, which increases the cost and complexity of the chip, or that a separate EPROM chip be provided.